This application is based on Patent Application No. 2000-35820 filed Feb. 14, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a method of measuring an inner stress state of a disk substrate and more specifically to a disk substrate""s inner stress state measuring method that can quantify with high precision a distribution of an inner state, such as stress and strain and flow orientation in a disk substrate material being microprocessed, visualize it multi-dimensionally and implement an appropriate quality control of the disk substrate.
2. Description of the Related Art
As there are growing demands on disks for higher density, lower cost and higher durability, the disk substrates are being required to have a higher quality. The rapid trend toward information society intensifies these demands for higher density, lower cost and higher durability. In optical disks in particular, the substrates of which are made mainly from plastics, the track width has been on the decrease for higher density and thus microfabrication substrates without distortion are being called for. In magnetic disks, too, there is a similar trend. It has been common practice that when recording data in a recording medium using an aluminum or glass substrate, position signals are written at the same time by a servo writer that records servo marks. In this way, address information is given for the magnetic head to track the recorded track.
This method, however, has a technical problem in enhancing the position accuracy. To deal with this situation, a method has been proposed which forms depressed or raised servo marks on the disk substrate by using the film deposition technology. At the same time there is a trend for making the substrate by molding plastic materials, such as thermoplastic polymers, for reduced weight and cost.
Fabrication of substrates by molding is advantageous in terms of cost because of its high productivity. As to the magnetic medium, however, because the magnetic head is in contact with the magnetic medium, as opposed to the optical disk which is kept out of contact with the head, and because the space between the magnetic head and the magnetic medium is only a few tens of nanometers, the substrate must have no warping, deformation or degraded durability. It is therefore important for improved performance, the prevention of performance deterioration and quality control to determine the inner state of the substrate, such as stress and strain and flow orientation, during the molding of the substrate.
As a means to that end, various methods have been proposed, which include a method of observing birefringence qualitatively using the conventional photoelasticity method (xe2x80x9cPhotoelastic Experimentationxe2x80x9d Published by Nikkankogyo Shimbun, a 1965 issue), an ellipsometer method as disclosed in Japanese Patent Laid-open No. 7-72013 (1995), a point measurement method as disclosed in Japanese Patent Laid-open Nos. 7-20040 (1995), 7-229828 (1995) and 10-267831 (1998), and a microscopic observation method as disclosed in Japanese Patent Laid-open No. 8-94444 (1996).
In the point measurement method, the measurement of distributions in the substrate takes time. In a method disclosed in Japanese Patent Laid-open No. 10-332533 (1998), although it can evaluate the birefringence distribution, this method has a problem in terms of precision when observing the inner state of the substrate because it observes the polarized light by using a color plate method. It is therefore necessary to automatically analyze the inner state (stress/strain and flow orientation, etc.) of the disk substrate.
Under these circumstances, the inventors of this invention conducted research and proposed a xe2x80x9cTwo-dimensional birefringence distribution measurement based on the phase shifting techniquexe2x80x9d (xe2x80x9cOPTICSxe2x80x9d October 1992, Vol. 21, No. 10, pp. 682 (34)-687 (39)) and a xe2x80x9cTwo-dimensional birefringence measurement using the phase shifting techniquexe2x80x9d (xe2x80x9cOPTICAL ENGINEERINGxe2x80x9d May 1994, Vol. 33, No. 5, pp. 1604-1609). According to the result of experiments conducted by using the basic principle for obtaining two-dimensional distribution of the birefringence, the present invention has made improvements and optimization especially on the method of measuring the inner stress state in the disk substrate.
In the molded disk substrate, a birefringence of the material itself and a birefringence phase difference caused by stress and strain developed in the material during the molding process induce a variety of problems in the way of manufacturing high precision substrates. Hence, there are growing demands for a measurement method that can measure quantitatively with high precision the inner stress state of the substrate and the distribution of the flow orientation during the molding of the substrate, and which can also perform quality control based on an automatic measurement.
The present invention has been accomplished to overcome the aforementioned drawbacks and provide a method of measuring an inner stress state in a disk substrate which can automatically analyze quantitatively the inner state of the disk substrate, i.e., stress-induced strains and a flow orientation state.
To achieve the above objective, the present invention according to a first aspect provides a measuring method which comprises the steps of: throwing a light wave onto a molded disk substrate, the light wave being produced by giving a phase difference to a principal axis of a linearly polarized light from a light source; image-processing a birefringent phase difference and a change in a principal axis direction according to an amount of change in polarization produced inside the disk substrate by the birefringence; and measuring multi-dimensionally an in-plane distribution of birefringence, such as an inner stress-induced strains in the molded disk substrate and a flow state of a resin being molded, using a phase shifting technique.
According to a second aspect, the invention provides a measuring method according to the first aspect, which includes: a retarder made from a liquid crystal element or electro-optic element to give a phase difference to the principal axis of the linearly polarized light emitted from the light source; a half-wave plate for rotating the principal axis direction of an elliptically polarized light; and a polarizer arranged in a direction perpendicular to the principal axis direction of the light wave rotated by the half-wave plate; wherein the elliptically polarized light is expanded into two dimensions by a lens system to produce image information and is thrown onto the disk substrate to detect an amount of change in the phase difference of an intensity of the transmitted light and motor-drive the half-wave plate and the polarizer, thus enabling an automatic measurement.
According to a third aspect, the invention provides a measuring method according to the first or second aspect, in which an electro-optic element is provided as a rotary polarizer for rotating the principal axis direction of the elliptically polarized light emitted from the retarder and the polarizer is fixed to detect the amount of change in the phase difference of an intensity of the transmitted light.
According to a fourth aspect, the invention provides a measuring method according to the first, second or third aspect, in which an optical lens is provided behind the polarizer to provide magnified or local viewing.
With the above construction, it is possible to automatically measure visually, quanitatively and with high precision the information on the internal state of the substrate, such as distribution of stress-induced strains and a flow orientation state, which develops inside the disk substrate during molding of a material or during microfabrication. This in turn allows for the quality control of substrates. Because optimum material and optimum molding conditions can be obtained, the measuring method of this invention is suited for producing high performance disk substrates.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.